The present invention relates to the control of a gas turbine or gas turbine power plane and, more particularly, to means for conditioning the generation of a blade path temperature limit signal to reflect the characteristics of the gas turbine. The present invention is equally suitable for employment in a primary control system of a gas turbine or plant or in a local maintenance or backup controller therefor.
In the local maintenance controller described in the above-referenced U.S. patent application Ser. No. 476,182, or in the primary control system for a gas turbine implemented power plant described in the above-referenced U.S. patent application Ser. No. 82,470, the particular signal generated to control fuel flow to the turbine, for purposes of varying speed or acceleration, is limited, inter alia, by a blade path temperature signal. In other words, a continued or increased flow of fuel, however otherwise correct, will be reduced if such continuation or increase will have or has the effect of raising blade path temperature above a prescribed limit. On the other hand, where blade path temperature is safely below such a limit, fuel flow could be increased above the quantity called for by the control system.
Unfortunately, as blade path temperature rises in response to increased fuel flow, the temperature increase and therefore the flow of fuel must be limited to a relatively slow rate so that the prescribed limit is not exceeded, i.e. can be safely approached without overshoot. Conversely, when cutting back on fuel flow and dropping away from the prescribed limit, the rate of reduced fuel flow can be relatively high, i.e., can be rapidly reduced.
As is apparent, the rate at which the signal regulating fuel flow can be changed, reflects a compromise between slow rate increases and fast rate decreases. In the case of the control system described in the aforementioned U.S. patent application Ser. No. 476,182, for example, the effects of reducing fuel flow where permissible are limited only by the inertial characteristics of the turbine whereas in increasing fuel flow, the limiting factor is the permissible growth rate of the control signal output or fuel scheduling signal.
However, in order to insure that any selective variation of the effects of the blade path temperature limit signal are not adverse, means for limiting excursions in the temperature limit signal to a predetermined level would be needed.
Consequently, there exists a specific need to provide a means of selectively varying the effect of a blade path temperature limit signal depending on whether overall control is proceeding away from or towards such limit consistent with the foregoing.